This invention relates generally to a magnetic disk, a method of producing the same, and a magnetic disk unit, and more particularly to a magnetic disk suitable for use as a thin film magnetic disk capable of achieving a high-density recording. More specifically, the invention relates to a magnetic disk having a surface nature suitable for flying characteristics of a magnetic head, CSS characteristics and sliding durability characteristics against a head adhesion, a method of producing such a magnetic disk, and a magnetic disk unit.
Conventionally, as shown in FIG. 5, in the production of a high-density recording magnetic disk in which a thin magnetic film is formed using a thin film formation technique such as sputtering, plating and ion-plating, a film of Ni--P several tens of .mu.m in thickness is formed by plating on a surface of an aluminum alloy substrate. Then, the opposite surfaces of the substrate are subjected to grinding or lapping and further to polishing using abrasive grains of various grain sizes, so that the surface of the substrate is finished into a surface roughness of 0.002 to 0.003 .mu.m Ra. Then, in order to avoid a head adhesion in CSS (Contact-Start-Stop) in a magnetic disk unit, texturing is applied to the magnetic disk substrate, thereby adjusting the surface roughness to 0.004 .mu.m to not more than 0.008 .mu.m Ra. Then, a magnetic film and a protective film are formed on this substrate, and a lubricating film is further formed thereon. As shown in FIG. 6, the magnetic disks 1 thus produced are combined with a magnetic head 2 to provide a magnetic disk unit.
On the other hand, in order to increase a recording capacity of the magnetic disk unit, a height H.sub.fly of flying of the magnetic head 2 off a magnetic disk surface 1 is very small, and is required to be not more than 0.1 .mu.m or to not more than 0.2 .mu.m. Therefore, the surface roughness of the textured surface is in the range of between not more than 0.004 .mu.m and a value close to the polished surface roughness (0.002 to 0.003 .mu.m Ra). Therefore, in order to reduce the flying height, it is necessary to decrease the surface roughness of the textured surface. However, in the magnetic disk of the type in which the thin film of the above layer structure is formed on the substrate surface, the head adhesion and the CSS tangential force are large, which has resulted in problems that a head support system 2' is damaged and that a disk drive motor fails to be rotated. In order to overcome these problems, there has been a demand for such a surface nature as is suitable for the surface roughness and the rugged surface (depressions and projections) of the magnetic disk surface, and therefore various shapes have heretofore been proposed.
For example, in Japanese Patent Unexamined Publication No. 55-117741, in order to improve wear resistance and lubricating properties of a magnetic (coating) film, depressions and projections are formed on the magnetic film by an abrasive tape, the depth of the depressions being about 0.1 .mu.m, and the size of the depressions and projections being about 2 .mu.m to about 3 .mu.m. In Japanese Patent Unexamined Publication No. 59-84348, fine depressions and projections are formed on a substrate by etching, thereby enhancing the durability of a lubricating film and CSS characteristics. In Japanese Patent Unexamined Publication No. 61-242334, polishing traces defining fine depressions and projections on a substrate are formed regularly in the direction of the circumference of the substrate. In Japanese Patent Unexamined Publication No. 62-248133, distal ends of fine projections and depressions are flat, and are disposed in a generally common plane, that is, generally at the same height. In Japanese Patent Unexamined Publication No. 1-192014, with respect to surface shapes of a substrate and a magnetic disk, the shape of fine depressions and projections is defined by means of a specified range of the average surface roughness and a specified range of a groove depth, thereby indicating the relation of the surface shapes with flying characteristics and sliding resistance characteristics of the magnetic disk.
One method of forming such a surface shape is a texturing method as shown in FIG. 8, in which fine grooves are formed in a Ni--P plated substrate of a magnetic disk in the direction of the circumference thereof, using abrasive grains which are most commonly used. In this processing method, the disk 11 is rotated, and contact rollers 6 are pressed against the opposite sides or faces of the disk 11 (in the direction indicated by arrow F) through abrasive tapes 3 and abrasive grains, and the abrasive tapes 3 are reciprocally moved radially of the disk 11 to form the fine grooves in the direction of the circumference of the disk. There has also been reported a method in which fine depressions and projections are formed by etching on a Ni--P plated substrate or a glass substrate for a magnetic disk, or a magnetic disk surface.
However, what is mentioned in the above reports is merely to roughen the surface in order to avoid the head adhesion and also to bring the projections on the surface into generally the same shape in order to prevent the head flying characteristics from being adversely affected, and these reports do not properly indicate the surface nature of the magnetic disk quantitatively in connection with the relationship of the head flying characteristics and the CSS tangential force which are necessary for the magnetic disk. Therefore, the optimum shape for the surface nature obtained by any of the above methods is not clear, and the conditions of the surface formation have been merely determined in a trial-and-error manner.
Namely, as described above, it is difficult to quantitatively indicate the relation of the conventional surface roughness Ra or Rmax with the head flying characteristics and the CSS tangential force.